1. Field of the Invention
The present invention relates to an apparatus for detecting a very weak magnetic field emanating from a human body, particularly from a person's heart or brain, or originating from ionosphere high above the surface of the earth or from an oil field deep under the ground, and any other weak magnetic field.
2. Description of the Prior Art
A dc superconducting quantum interference device (abbrebiated as "dc-SQUID") or a radiofrequency superconducting quantum interference device (abbrebiated as "rf-SQUID") using Josephson junctions has been hitherto used in measuring a very weak magnetic field with increased sensitivity. Its output signal representing the magnetic flux of such a weak magnetic field, contains a non-zero constant, which is called "dc offset". Where the magnitude of the magnetic flux rather than the amount of vaiation of magnetic flux is to be determined, it is necessary to carry out null flux calibration by subtracting the dc offset value from the output signal appearing at the output terminals of the SQUID device. The null flux calibration, however, is very difficult to perform.